1. Field of the Invention:
As is well known, various surface characteristics tend to vary the resistance offered by the skin of an aircraft in flight to an airstream flowing thereover. For example, rivet heads, surface textures, finishes and the like, all tend to offer resistance to an airstream as it is caused to flow thereover. Such resistance, herein referred to as skin friction, creates drag, herein referred to as skin-friction drag, which significantly tends to impair the overall operational efficiency of an aircraft.
The instant invention generally relates to a device particulaly suited for use in studying shear force on aircraft in flight and more specifically to an improved skin friction measuring device which is of a large rugged construction, economic to fabricate, simple to operate and characterized by small displacement, preferably on the order of 0.005 to 0.010 inches. The skin friction measuring device which embodies the principles of the instant invention is particularly adapted to be mounted aboard subsonic aircraft and employed in obtaining research and design data, as it relates to the skin friction characteristics of the aircraft in flight, however, it is to be understood that the invention is equally useful in wind-tunnel environments.
2. Description of the Prior Art:
During the course of a preliminary search conducted for the invention, a patent to Hafner et al No. 4,112,752 was discovered. This patent discloses a force measuring device adapted to be moved between hydrostatic bearings and employs strain gauges for providing outputs indicative of the force components exerted by a flowing medium.
Additionally, U.S. Pat. No. 2,935,870 was discovered. This patent discloses a skin friction measuring device sensitive to acceleration forces and is adapted to support segments of the surface of a missile.
A patent to Kistler U.S. Pat. No. 3,304,775 also was discovered. This patent discloses an aerodynamic drive measuring device. In operation, movement through a medium of a vehicle in which the device is mounted produces a driving force on a sensing element which, in turn, is detected as a change in capacitance in a sensing circuit.
A patent to MacArthur U.S. Pat. No. 3,383,914 was discovered which discloses a skin friction transducer wherein a shear sensitive element is supported for a strap connected to a piezoelectric element. In operation, a flow is directed across the surface of the sensing element causing the piezoelectric element to deflect for producing a voltage, the magnitude of which is proportional to the deflection.
A patent to Padera U.S. Pat. No. 3,460,383 also was discovered. This patent discloses a control surface for aerodynamic or hydrodynamic models having flexured areas so configured as to enable accurate utilization of strain gauges or the like on the exterior of control surfaces.
Finally, a patent to Bush U.S. Pat. No. 3,714,524 was discovered. The patent to Bush discloses a skin friction measuring apparatus. However, the patented apparatus differs substantially in structure from the invention hereinafter more specifically described.
While, as should now be apparent, various devices have been proposed for use as skin friction measuring devices, it has in practice been found that the previously employed skin friction measuring devices simply fail to provide the data necessarily required for the design of operationally efficient aircraft. As a consequence of this inadequacy, tests have been delayed, inaccurate data obtained, and significant expense have been encountered in conducting routine experiments, to say nothing of the costs encountered in the repair and maintenance of the devices previously employed.
It is, therefore, the general purpose of the instant invention to provide a simple, rugged, practical and economically feasible skin friction measuring device which is particularly adapted to be employed in a test station environment for routinely studying the resistance offered to fluids caused to flow across the skin of an aircraft in flight, herein simply referred to as a skin friction measuring device.